The present invention relates generally to a paperless gypsum/fiber board with improved impact resistance and to a process for making such a gypsum/fiber board. More particularly, the present invention relates to a multi-layer gypsum/fiber board having a fiberglass mesh embedded in the backside to provide improved impact resistance.
Conventional gypsum wallboard or panel is typically manufactured from a plaster slurry wherein a wet slurry of calcium sulfate hemihydrate, generally referred to as calcined gypsum, is placed between two layers of paper and the slurry is allowed to set. The set gypsum is a hard and rigid product obtained when the calcined gypsum reacts with water to form calcium sulfate dihydrate. Calcined gypsum is either calcium sulfate hemihydrate (CaSO4xc2x7xc2xd H2O) or calcium sulfate anhydrite (CaSO4). When calcium sulfate dihydrate is heated sufficiently, in a process called calcining, the water of hydration is driven off and there can be formed either calcium sulfate hemihydrate or calcium sulfate anhydrite, depending on the temperature and duration of exposure. When water is added to the calcined gypsum to cause the gypsum to set, in essence, the calcined gypsum reacts with water, and the calcined gypsum is rehydrated. In typical gypsum wallboard, the two layers of paper contain the slurry and provide the strength required in installation and use. The wallboard is cut into discrete lengths to accommodate subsequent handling and then dried in heated dryers until the board is completely dry. The bending strength of the wallboard depends largely on the tensile strength of the paper. The set gypsum serves as the core and accounts for fire resistance and can be modified for various applications. The paper determines the nature of the application for the board and the surface treatment that may be used on the board.
Although paper-covered wallboard has many uses and has been a popular building material for many years, the prior art has recognized that for certain applications it would be advantageous to provide a gypsum panel that did not rely on paper surface sheets for strength and other properties. Several prior art fiber-reinforced gypsum panels are as follows:
U.S. Pat. No. 5,320,677 to Baig, which is incorporated by reference herein in its entirety, describes a composite product and a process for producing the product in which a dilute slurry of gypsum particles and cellulosic fibers are heated under pressure to convert the gypsum to calcium sulfate alpha hemihydrate. The cellulosic fibers have pores or voids on the surface and the alpha hemihydrate crystals form within, on and around the voids and pores of the cellulosic fibers. The heated slurry is then dewatered to form a mat, preferably using equipment similar to paper making equipment, and before the slurry cools enough to rehydrate the hemihydrate to gypsum, the mat is pressed into a board of the desired configuration. The pressed mat is cooled and the hemihydrate rehydrates to gypsum to form a dimensionally stable, strong and useful building board. The board is thereafter trimmed and dried. The process described in U.S. Pat. No. 5,320,677 is distinguishable from the earlier processes in that the calcination of the gypsum takes place in the presence of the cellulosic fibers, while the gypsum is in the form of a dilute slurry, so that the slurry wets out the cellulosic fibers, carrying dissolved gypsum into the voids of the fibers, and the calcining forms acicular calcium sulfate alpha-hemihydrate crystals in situ in and about the voids.
U.S. Pat. No. 5,135,805 to Sellers et al, describes a water resistant gypsum product that may be a xe2x80x9cfacelessxe2x80x9d product, i.e. it may not include a facing sheet of paper, fiberglass mat or similar material. The gypsum products described by U.S. Pat. No. 5,135,805 typically contain reinforcing fibers, for example, cellulosic fibers, such as wood or paper fibers, glass fibers or other mineral fibers and polypropylene or other synthetic resinous fibers. The reinforcing fibers can be about 10 to about 20 wt. % of the dry composition from which the set gypsum product is made. The density of such a product is typicality within the range of about 50 to about 80 pounds per cubic foot.
U.S. Pat. No. 5,342,566 to Schafer et al, which is incorporated by reference herein in its entirety, refers to a method of producing fiber gypsum board comprising the steps of mixing in a preliminary mixing step predetermined amounts of fibers and water respectively, to form a mixture of wetted, loose fibers; mixing in a mixing step the wetted fibers with a predetermined amount of dry calcined gypsum; premixing an accelerator with one of the components of dry calcined gypsum, fiber and water; promptly laying the mixed composition into a mat; immediately degassing the mat in a first compression step, adding a predetermined amount of water onto the resultant mat; and immediately compressing the mat to form a board composed of bonded fibers and gypsum. This process may be used to produce a homogeneous board which is preferably a gypsum board reinforced by fiber, such as paper fiber, wherein several layers of board forming materials are placed on each other before the board is fully formed, pressed, and dried and wherein each of the layers is identical in composition. Schafer et al specifically describes the formation of a three layer board wherein the central, core layer has a composition which differs from the composition of the outer layers.
Carbo et al Provisional Application Serial No. 60/073,503, describes a multi-layer, paperless gypsum/fiber board and a process for making such a three layer gypsum/fiber board wherein the central, core layer has a composition which differs from the composition of the outer layers, all of which is incorporated by reference herein in its entirety.
Prior art gypsum/fiber boards have been modified by adhering a layer of mesh to the back of the board, in order to provide improved impact resistance. While such modified boards had improved impact resistance, the production rates of such board were low, the energy costs were higher, the materials cost increased, the labor cost increased, because of the need to laminate the gypsum boards to the mesh in separate processes that have control problems with respect to the lamination process and the problem of blocking between panels. The process of lamination can be difficult with any variation of thickness of the panel being covered with a mesh or solid reinforcement. With variation in thickness or profile the lamination process is a problem with respect to maintaining constant pressure across the panel. The problem of blocking panels which stick to one another is a significant problem when laminating mesh""s on surfaces as the glue can actually glue the stacked panels to one another in addition the mesh on the surface.
It is the object of the present invention to provide a fiber-reinforced gypsum board that has improved impact resistance that avoids many of the problems of the prior art gypsum/fiber boards. More particularly, it is the object of present invention to provide a multi-layer gypsum/fiber board having a fiberglass mesh embedded in the backside to provide improved impact resistance as determined by Soft Body Impact Resistance according to ASTM E695 and by Hard Body Impact Resistance according to USG method as documented in independent reports HPWLI #7122 and HPWLI #7811-02. Embedding a reinforcing mesh to the gypsum/fiber board, in accordance with the present invention, provides many advantages including high production rates; better product aesthetics, integral consolidation of reinforcing mesh in board and reduced product cost. Embedding a reinforcing mesh also improves the handling properties of the board and reduces the tendency of the board to block (adhere to adjacent boards when horizontally stacked). The product of the present invention can include a flush mesh which does not mark up the face of the panel on which it is stacked, and improved retention of the reinforcement in the panels as it is protected from wearing and rubbing on the surface. Another product benefit is the tensioning of the mesh in the product to provide enhanced stiffness to the panel. In terms of the process the present invention eliminates the need to transport the panels to secondary operations, and allows the reinforced panel to be produced on a standard gypsum fiberboard line.
The present invention relates generally to a paperless gypsum/fiber board with improved impact resistance, and to a process for making such a gypsum/fiber board. The term xe2x80x9cpaperlessxe2x80x9d gypsum/fiber board, as used herein, is intended to distinguish the fiber-reinforced gypsum panels to which the present invention relates from conventional prior art gypsum panels, which are referred to as xe2x80x9cwall boardxe2x80x9d or xe2x80x9cdry wallxe2x80x9d which have at least one surface comprised of paper, including xe2x80x9cwall boardxe2x80x9d or xe2x80x9cdry wallxe2x80x9d having some form of fiber-reinforcement in the core.
The paperless gypsum/fiber board having improved impact resistance is produced by embedding a reinforcing mesh, preferably a flexible fiberglass mesh, in the back side of a multi-layer gypsum fiber board. In the process, the mesh is fed into the forming area of the panel before the panel is pressed prior to drying.
It is to be understood that the foregoing general description, and the following detailed description, are exemplary and explanatory only and are not restrictive of the invention, as claimed.